This invention relates to electroluminescent devices. More particularly, it is concerned with thin film electroluminescent devices in which an active element of a thin layer of phosphor material is sandwiched between two dielectric films.
Thin film electroluminescent devices are employed for various forms of displays. Typically the devices employ a transparent substrate having on one surface a very thin conductive electrode which is substantially transparent. This first electrode is covered with an insulating layer. A layer of a suitable phosphor material overlies the insulating layer. The phosphor layer is covered with another insulating layer, and a second conductive electrode of an appropriate pattern is formed on the second insulating layer. Under operating conditions a voltage is applied between the two electrodes causing the portion of the phosphor layer between the electrodes to luminesce, thus providing a visible pattern when viewed through the transparent substrate.
Typically the phosphor layer is a host of zinc sulfide containing an activator, frequently manganese. Light generated in this phosphor layer by the voltage across the electrodes passes through the layer of insulating material and the conductive electrode to be viewed as it passes through the transparent substrate. Some of the light generated in the phosphor layer passes through the other insulating layer to the second conductive electrode. When the second electrode is of a reflecting material, such as aluminum, the light striking the second electrode is reflected back through the layers of the device passing through the transparent substrate as visible light. Since in conventional devices the layers are of planar geometry, some of the light generated in the phosphor layer is trapped within the phosphor layer by internal reflection at the interfaces of the phosphor layer and the two layers of insulating material, and consequently does not become visible.